AKA: Consider supporting SET default_transaction_isolation in the postgresql wire API.

Calling set_isolation_level to change the isolation level of a psycopg2 connection to cockroach fails with this error: ERROR: unknown variable: "DEFAULT_TRANSACTION_ISOLATION". That happens because psycopg2 sends SET default_transaction_isolation TO '<the isolation level>' but cockroach doesn't support that syntax in the postgres wire API (it does in the CLI).

The easiest way to reproduce this it to connect to a cockroach server using psql (psql postgres://localhost:26257) and executing the command at the prompt, e.g. SET default_transaction_isolation TO 'serializable'.

Note: the problem only shows up via psycopg2 if you set the isolation level to something other than the default, i.e. ISOLATION_LEVEL_READ_UNCOMMITTED or ISOLATION_LEVEL_READ_COMMITTED because psycopg2 won't send the command if the isolation level doesn't need to change.